Threshold analysis for transverse instabilities in photorefractive counterpropagating two-wave mixing through reflection gratings is performed. A numerical algorithm for the treatment of wave equations in this geometry is developed, displaying the emergence of running transverse waves. They appear above a threshold in the applied electric field, and their transverse wave number and oscillation frequency agree well with the values predicted by stability analysis. (C) 1991 Optical Society of America.
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